1. Field of the Invention
The present invention relates to an optical writing head and an image forming apparatus. More particularly, the present invention relates to an optical writing head designed to cause a plurality of light emitting devices to project rays of light onto a target irradiation surface by means of a lens array to form image formation spots, and an image forming apparatus using the same. Such an optical writing head finds applications in the fields of electrophotographic copying machines, printers, and facsimile machines.
2. Description of the Related Art
Known optical writing heads to be used in electrophotographic copying machines include an array of light emitting devices, such as LEDs, and a rod lens array formed by arranging a plurality of rod lenses having a refractive index distribution between the light emitting device array and a photosensitive drum that operates as an image carrier.
A flux of light that is modulated according to an image signal is emitted from each of the light emitting devices and converged to a spot on the surface of the photosensitive drum by the rod lens array to record an image.
Such optical writing heads are required to have a structure that can be more easily assembled.
When the rod lens array and the light emitting device array are displaced relative to each other so as to show a shift from a preset value, the shape of the entrance pupil and that of the exit pupil change to thereby change the quantity of light and the shape of a light spot formed on the surface of the photosensitive drum.
Then, as a result, an uneven density and color changes appear on the recorded image. To avoid such a problem, the rod lens array and the light emitting device array need to be accurately aligned relative to each other.
U.S. Pat. No. 7,486,306 (“the '306 patent”) proposes a color image forming apparatus that can position the light spots of the component colors of the color of a light spot on the surface of a photosensitive drum so as to make the shapes of the light spots change to show the same shape in a main scanning direction as a technique for reducing the color change attributable to a change in the shape of the light spot.
The known technique in the '306 patent is provided to reduce the color change by making the shapes of the light spots of the component colors change to show the same shape and, hence, is accompanied by a problem as described below.
To make the light spots of the component colors show the same shape, the shapes of light spots of the component colors need to be adjusted so as to make them show the same and equal change.
For this purpose, the position of the light emitting device array and that of the rod lens array need to be accurately aligned relative to each other. Then, the optical writing head assembling process requires an adjustment unit or an adjustment step for accurate alignment. This makes the optical writing head costly.